1. Field of the Invention
The present invention relates to a technique capable of irradiating a large area with a laser beam having high uniformity, and also the invention relates to an application method thereof.
2. Description of the Related Art
In recent years, a technique of laser annealing to an amorphous semiconductor film or a non-single crystal semiconductor film (a semiconductor film which is not single crystal but has crystallinity of polycrystal, microcrystal, etc.) formed on an insulating substrate such as glass to crystallize the film or improve its crystallinity, has been widely studied. As the semiconductor film, a silicon film is often used. As compared with a quartz substrate which has been hitherto often used, a glass substrate has such merits that it is inexpensive and is rich in workability, and a large substrate can be easily formed. This is the reason why the foregoing study is carried out. Besides, the reason why a laser is preferably used for crystallization is that the melting point of the glass substrate is low. The laser can give high energy to only a non-single crystal film without greatly changing the temperature of the substrate.
Since a crystalline silicon film formed by laser annealing has a high mobility, a thin film transistor (TFT) is formed by using this crystalline silicon film, and for example, it is actively used for a monolithic liquid crystal electro-optical device in which TFTs for pixel driving and for driver circuits are formed on one glass substrate. Since a crystalline silicon film comprises a number of crystal grains, it is called a polycrystal silicon film or a polycrystal semiconductor film.
A method in which a pulse laser beam of an excimer laser or the like having high output is processed by an optical system so that a spot of several cm square or a line of several hundred .mu.m width.times.several tens cm is formed on a surface to be irradiated, and the laser beam is made to scan (irradiation position of the laser beam is relatively moved to the irradiated surface) to make laser annealing, is superior in mass productivity and excellent in technology. Thus, this method is used by choice.
Particularly, when a linear laser beam is used, differently from the case of using a spot-like laser beam which requires back-and-forth and right-and-left scanning, laser irradiation to the whole irradiated surface can be made by scanning in only the direction normal to the line direction of the linear laser. Thus, high mass productivity can be obtained. The reason why scanning is made in the direction normal to the line direction is that it is the most effective scanning direction. By this high mass productivity, at present, laser annealing using the linear laser beam has become the mainstream.
When laser annealing is made to the non-crystal semiconductor film by scanning of the processed linear pulse laser beam, some problems have occurred. One of especially serious problems among them is that laser annealing is not uniformly carried out to the whole surface of the film. When the linear laser beam is used, a phenomenon in which stripes are formed at overlapped portions of the beam becomes noticeable, and semiconductor characteristics of the film are remarkably different for each of these stripes.
FIG. 1 shows the state of these stripes. When the surface of a silicon film after laser annealing is observed, these stripes appear according to the degree of reflection of light.
FIG. 1 shows the state in the case where XeCI excimer laser with a wavelength of 308 nm was made a linear laser beam extending in the right-and-left direction on the paper surface, and irradiation was made while this laser beam scanned a film in the direction from the upper portion of the paper surface to the lower portion.
In the case where an active matrix type liquid crystal display is manufactured by using a silicon film in which a stripe-like pattern as shown in FIG. 1 appears, there occurs a disadvantage that this stripe appears directly on the screen.
Although this problem has been improved by improving a non-single crystal semiconductor film of an object to be irradiated with laser, or by narrowing a scanning pitch (interval of adjacent linear laser beams) of the linear laser, it has been still insufficient.
In general, in the case where a linear laser beam is formed, an original rectangular beam is made to pass through a suitable optical system and is processed into a linear shape. Although the aspect ratio of the rectangular beam is about 2 to 5, for example, by an optical system shown in FIG. 2, it is transformed into the linear beam having an aspect ratio of 100 or more. At that time, the optical system is designed such that the distribution of energy in the beam is also homogenized at the same time.
The apparatus shown in FIG. 2 has a function to emit a laser beam, as a linear beam, from a laser beam generating unit 201 (in this state, the shape of the beam is substantially rectangular) through optical systems 202, 203, 204, 206, and 208. Incidentally, reference numeral 205 denotes a slit, and 207 denotes a mirror.
Reference numeral 202 denotes an optical lens serving to divide a laser beam in one direction, and a cylindrical lens group (also referred to as a multicylindrical lens) is used. The divided many beams are overlapped and homogenized by the cylindrical lens 206.
This structure is required to improve the strength distribution in the laser beam. The cylindrical lens group 203 also divides the laser beam in another direction, like the foregoing cylindrical lens group 202, and the divided beams are overlapped and homogenized by the cylindrical lenses 204 and 208.
That is, the combination of the cylindrical lens group 202 and the cylindrical lens 206 has a function to improve the strength distribution in the line direction of the linear laser beam, and the combination of the cylindrical lens group 203 and the cylindrical lenses 204 and 208 has a function to improve the strength distribution in the width direction of the linear laser beam.
Here, with respect to the width direction, the two cylindrical lenses 204 and 208 are used to make finer in the width direction of the linear laser beam on the irradiated surface. According to the width of the linear laser beam, the number of optical systems for overlapping is made one, or made three or more.
The optical system serving to homogenize the energy distribution in the laser beam is referred to as a beam homogenizer. The optical system shown in FIG. 2 is also one of beam homogenizers. The method of homogenizing the energy distribution is such that after the original rectangular laser beam is divided by the cylindrical lens groups 202 and 203, the divided beams are shaped and overlapped by the cylindrical lenses 206, 204 and 208 to homogenize them.